In a microwave oven, it is desirable to elevate the food a short distance from the floor of the cavity. This spaces the food from the electric component null on the conductive surface of the floor. Also, it permits microwave energy to enter the food from all sides. In the prior art, food has been elevated by a relatively thin microwave transparent tray supported at the edges of the conductive floor which is recessed downwardly in the central region.
While positioning a shelf at some level above the bottom tray may have some advantage in particular microwave cooking applications by increasing the total food supporting area, the use of an elevated shelf is of great importance in a thermal oven where the heat should be conducted into the food from all sides, including the bottom. Accordingly, in a cavity used for both microwave and thermal heating, it is most desirable to have both the bottom transparent tray and an elevated shelf thereabove.
In a prior art combination microwave and convection heat oven, a shelf defined by a metal rod rack was used. The open spaces in the rack permitted circulation of the hot air around the cavity. With this construction, it was desirable to use metal for the rods to provide sufficient strength. It was desirable to be able to change the height of the rack or remove it altogether for various cooking applications. Also, it was desirable to be able to slide the rack forward to assist in handling of the food dish. Accordingly, pairs of grooved microwave transparent shelf support brackets were screwed in vertical alignment on opposing sides of the conductive cavity. The side perimeter rods of the rack were inserted into respective horizontal grooves in the brackets thereby facilitating forward and backward movement. It was found that under unusual circumstances, the support brackets would melt or deform due to overheating. Also, there were isolated instances of arcing between the metal rack and the support bracket screws or between the metal rack and the dielectric material of the support bracket. Such arcing may occur to the surface of insulating material at microwave frequencies because the displacement current in the dielectric completes the circuit.